The present invention relates to a self-actuated nuclear reactor shutdown system wherein a control rod suspended by a temperature sensitive electromagnet (TSEM) is automatically separated or detached from the TSEM and is inserted into a reactor core to effect an emergency shutdown of the reactor when the temperature of a coolant rises in an extraordinary manner. In the present invention, a temperature sensitive magnetic material (TSMM) is located in the wall of wrapper tubes adjacent to a control rod guide tube and constitutes a part of a magnetic circuit of the TSEM. Therefore, the response to the thermal transient can be improved and the stability in the actuation of the TSEM can also be expected.
The reactor shutdown system of the present invention can be utilized for fast breeder reactors, light water reactors and the like.
Nuclear reactors of various types are provided with some back-up control rods to be inserted into a reactor core when anything unusual occurs, in addition to control rods for power control, in order to enhance the reliability of the reactor shutdown. For a reactor shutdown system, it has been proposed that a TSEM in which a TSMM is provided as a part of the magnetic circuit thereof is installed inside the reactor core so as to implement the retention and separation of the control rod. As for the TSEM, a ferromagnetic material which has a suitable Curie point is employed to break the magnetic circuit by decreasing saturation flux density when the temperature of the TSEM is close to the Curie point.
During a normal operation of the nuclear reactor, the control rod is suspended at the upper part of the control rod guide tube by the TSEM. When the temperature of the coolant flowing through a fuel assembly rises due to an extraordinary accident, the TSMM transforms from a ferromagnetic substance to a nonmagnetic one. Therefore, the magnetic circuit of the electromagnet is broken at the TSMM and the TSEM no longer exerts its holding force. Consequently, the control rod can be spontaneously unlatched from the TSEM and inserted into the reactor core and the reactor is shut down.
It has been proposed in the prior art to provide some structure to introduce hot coolant from the fuel assembly to the TSEM such as, for example, a coolant introduction pipe, in order to prospectively obtain a quick response of the TSEM to thermal transient of the coolant.
Such a prior art structure as described above, however, employs a complicated mechanism for introducing the coolant flowing through an adjacent fuel assembly into the control rod guide tube, and cannot provide high reliability.
Besides, it is expected that a response time will be longer when the flow rate of the coolant decreases, because the flow rate of the coolant at a high temperature introduced into the control rod guide tube cannot be increased in comparison with the mass of the TSMM. Moreover, in the structure wherein the control rod guide tube is used as a part of the magnetic circuit, the vertical relative position between the electromagnet and the control rod guide tube must always be controlled with high precision, and this impairs the intrinsic feature of the self-actuated shutdown system which is to actuate without any external control.